Photocatalysts are a unique class of semiconductor materials that absorbs light and catalyzes a reaction. They are currently widely studied in the area of environment and energy applications such as solar cells, photocatalytic hydrogen production (“water splitting”), waste water treatment, and chemical sensing. The current mission for the search for an ideal photocatalyst promises to be a difficult quest. The main requirements of these materials are that they possess appropriate electronic structures (band gap, band energy positions) for a maximum efficiency and productivity, be chemically stable in the application environment (accordingly non-toxic), and be reproducible in large quantity at low cost. It has been proven difficult to find semiconductors that satisfy the above criteria. For example, although many metal chalcogenides, such as sulfides and selenides, have low band-gaps and respond to or absorb visible light, they are easily corroded and often toxic, while the cost of production is substantial. In contrast, metal oxide semiconductors are environmentally benign, cheap to produce, and some of them are very stable in diverse environments. However, they are wide-band gap materials in their pristine state. Even the most successful oxide semiconductor in environmental applications, titanium dioxide (TiO2), possesses a band gap of 3.2 eV (in its photocatalytically-preferred anatase form), making it sensitive only to UV light (λ≦388 nm). Since UV light only covers 5% or less of the solar spectrum, it makes the semiconductor photocatalyst inefficient.
As in photocatalysis, one of the major efforts in recent years has been to render titanium oxide responsive to visible light irradiation so as to increase its ability to harness the solar spectrum. This is achieved by the addition of dopants; traditionally metal cations, but more recently also non-metals or anions such as carbon, nitrogen, and sulphur. The latter is an attempt to reduce the width of the forbidden band, for example by raising the valence band or lowering the conduction band [Serpone, N, Journal of Physical Chemistry B (2006) 110 24287-24293], ideally without compromising redox functionality [Mrowetz, M, et al., Journal of Physical Chemistry B (2004) 108, 17269-17273].
So far the results on metal doping have been contradictory: both increase and decrease of photocatalytic activity have been reported [Riishiro, R, et al., Physical Chemistry Chemical Physics (2005) 7, 2241-2245; Fresno, F, et al., Journal of Photochemistry and Photobiology A-Chemistry (2005) 173, 13-20; Colmenares, JC, et al., Applied Catalysis A—General (2006) 306, 120-127]. The decrease may occur because the dopant behaves as a recombination centre and the energy levels formed by low-level doping can be discreet and lead to lower mobility of charge carriers. Up to today, the scientific community tends to support the view that metal doping is not a practical way to improve the catalytic activity of titanium oxide. This is not only due to ambiguous or unpredictable performance data, but also due to corrosion problems and prohibitive cost for some cationic dopants. Anion or non-metal doping has received increasing attention in recent years [Serpone, 2006, supra; Asahi, R, et al, SCIENCE (2001) 293, 269-271; Cong, Y, et al., Chemistry Letters (2006) 35, 800-801; Di Valentin, C, et al., Chemistry of Materials (2005) 17, 6656-6665; Umebayashi, T, et al., Applied Physics Letters (2002) 81, 454-456; Sakthivel, S, & Kisch, H, Angewandte Chemie Int. Ed. (2003) 42, 4908-4911; Sakthivel S, & Kisch H, Photocatalytic and photoelectrochemical properties of nitrogen-doped titanium dioxide, ChemPhysChem (2003) 4, 487-490; Tielens, F, et al., Journal of Electro-Analytical Chemistry (2005) 581, 216-223] as a promising alternative approach. Unfortunately, from a materials processing viewpoint, it is more difficult to achieve proper (homogeneous bulk) doping with anions as compared to cations. Successful procedures are generally vacuum-based, for example physical vapour deposition, chemical vapour deposition and ion implantation; all far too expensive for commercial viability.
Accordingly it is an object of the present invention to provide a material that can be used as a photocatalyst under visible light, overcoming at least some of the above discussed draw-backs.